The present invention relates to a printer or similar image forming equipment selectively operable in a monocolor print mode or a multicolor print mode.
Conventional image forming equipment includes a printer which incorporates a plurality of developing units each storing a toner or similar developer of a particular color so as to form a desired monocolor or multicolor image. With this type of image forming equipment, it is a common practice to produce, for example, a multicolor image by transferring toners of different colors superposed one above another to a recording medium and then melt and thereby fix them on the medium. Such a procedure is successful in rendering an image in bright colors. A color OHP of the kind dealing with multicolor images has to melt and mix the toners of different colors sufficiently in a uniform distribution by a fixing unit, so that the transmittance and the gloss of a recording medium may be enhanced. The fixing unit, therefore, needs an amount of heat great enough to melt a transparent color toner or color toners of different colors sufficiently. Assuming a black-and-white image and a color image, the acceptable range of fixing heat differs from a black toner to color toners and from a monocolor print mode which uses only a black toner or a toner of any other color to a multicolor print mode which transfers toners of different colors lying one above another.
To change the amount of fixing heat, the fixing unit may have the pressure to be exerted by a fixing roller thereof, the fixing temperature, the fixing speed or similar factor thereof changed. However, changing the fixing pressure is not practicable without resorting to a special pressure changing mechanism which adds to the cost. Moreover, a high pressure and a high torque are needed when it comes to a multicolor image. Changing the fixing temperature is disadvantageous in that when the temperature is changed, the operator has to simply wait until the temperature reaches a predetermined one, resulting in poor operability. By contrast, changing the fixing speed is free from such drawbacks, and this scheme is disclosed in, for example, Japanese Patent Publications 22226/1984 and 1746/1986, Japanese Patent Laid-Open Publication No. 188673/1984, an Japanese Utility Model Laid-Open Publication No. 192158/1984.
The fixing speed may be changed depending on the operation mode, i.e., a monocolor print mode or a multicolor print mode, as taught in some of the above-mentioned documents. A problem with this scheme is that two or more processing speeds are necessary in order to render the speed of the entire equipment variable and, therefore, processing conditions cannot be set with ease. Moreover, in a multicolor print mode, the printing speed is lowered. Alternatively, the fixing speed may be changed after the transfer of an image, with the processing speed maintained constant. A prerequisite with this approach is that a recording medium be present between an image transfer position and a nip position defined in the fixing unit, i.e., the distance which a recording medium is to travel from the transfer position to the fixing unit be greater than the length of the medium. This increases the overall size of the equipment.
As discussed above, none of the conventional approaches of the kind changing the fixing speed is fully satisfactory.